This application addresses nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors or electric field based capacitive sensors. Characterization of bulk material condition includes (1) measurement of changes in material state, i.e., degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from aggressive grinding, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or with single or multiple cracks, cracks or stress variations in magnitude, orientation or distribution.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
A typical application of these techniques is the inspection of high-strength steel components with the goal of measuring applied and residual stresses and detecting early stage fatigue damage. Highly stressed aircraft components, such as landing gear components, require the use of steels such as 4340M and 300M heat treated to very high strength levels. The integrity of these components is critical to the safe operation of aircraft and for maintaining readiness of military aircraft. However, unintentional loading of these components, such as a hard landing or during towing or taxiing, can impart residual stresses that compromise the integrity of the component.
Existing magnetic/electromagnetic, diffraction, ultrasonic and other methods for assessment of residual stresses in steel components or monitoring of applied stress over wide areas are not yet practical or cost-effective. Typically, discrete strain gages are mounted directly onto the material under test (MUT). However this requires intimate fixed contact between the strain gage and the MUT and individual connections to each of the strain gages, both of which limit the potential usefulness for monitoring stress over large areas. Furthermore, strain gages are limited in durability and do not always provide sufficient warning of gage failure or malfunction.
Correlations between magnetic properties and stresses in ferromagnetic materials have been studied for over 100 years, as reviewed by Bozorth. Magnetostriction effect data suggests that, depending on the magnitude and sign of the magnetostriction coefficient, correlation between stress and magnetic permeability within certain ranges of the magnetic field should be present. However, attempts to use conventional inductive, i.e., eddy-current sensors for assessment of residual stresses as well as for a number of other applications have shown serious limitations, particularly for complex geometry components. This is typical of many inspections where direct inspections of the component material may only provide limited observability of the property of interest.